Note: Descriptions are shown in the official language in which they were submitted.
3~
~ACKGROUND OF THE I~ENTTON
Field of the Inventlon
This invention relates to a process for removal
of solids from a liquid containing suspended solids by
flowing the liquid through a filter bed of discontinuous
polyurethane particles.
Descr~tion of the Prior Art
In the field of liquid and wastewater treatment,
filtration has long been a major method of removing sus-
pénded solids from liquid streams. In wastewater treat-
ment applications in particular, the presence of suspended
solid materials is frequently a major process problem
and filtration has commonly been employed as a means of
reducing and/or removing suspended solids from st~eams
such as municipal sewage and wastewater intended for re-
cycle use. In such applications, clownflow and upflow
sand filters and dual or mixed medla filters ha~e been
widely ernployed and have in general been shown to be
cost effective and efficient in use. Nonetheless, work
by practitioners in the field has shown that sand and
mixed media filters are in general effective in removing
suspended solids, but only under limited solids loading
conditi~ns. In general, solids concentration of the
liquid stream entering the filter must be below about
100-200 milligrams/liter. ~ ~ ~
6.153~
At suspended solids concentration values above this level,
the filtration bed is susceptible to clogging and high
pressure drop across the bed.
In recent years, the operatlon of filtration
has been somewhat improved in the foregoing applications
by the use of polyurethane as a filtration medium. In
general practice, polyurethane foam is cut up into _mall
pieces and placed into a retaining vessel to form the
filtration bed. Wastewater or other liquid containing
suspended solids is then flowed through the bed ~ith re
sulting deposition of suspended solids on the polyurethane
particles. The suspended solids thus remain on the poly-
urethane particles and the liquid from which the sus-
pended solids has been deposited subsequently passes
through the bed and is discharged as liquid depleted in
suspended solid contaminants.
In general, the use of polyurethane foam as a
flltration medium provides numerous operating advantages
over beds employing sand or conventional mixed media
filtration materials, including higher solids capacity,
lower pressure drop head losses, higher resistance to
clogging and removal of numerous soluble organic contam-
inants from the liquid stream.
Despite its numerous operating advantages,
however, regeneration of the polyurethane foam which has
become loaded wlth suspended solids in the filtration
operation has continued to pose a severe operating problem.
Various methods have been contemplated for regeneration
of the polyurethane ~oam filtration bed which has become
3o ~t least partially loaded with suspended solids during the
- 3
lV~ JUL
31~
flltration step, including back-washing with a solids-
depleted liquid stream, such as is commonly employed in
conventional sand and mixed media filtration systems.
The use of back washing in a polyurethane foam filtration
bed poses a specific problem due to the low density of
the polyurethane foam filtration medium. Since poly-
urethane has a true density which may be as low as 1 - 2
pound/foot , it ls generally desirable in normal operation
to flow liquid containing suspended solids do~mwardly
through the bed of polyurethane foam material. Back
washing of such a filtration bed, which involves counter-
currently flowing a stream of clean liquid upwardly through
the filtration bed, in general requires such ~gh flow
velocities through the bed as to wash particles of poly-
urethane foam out of the bed or else causessuch disruption
of the bed as to leave large void spaces therein which
contribute to bypassing and other disadvantageous per-
~ormance behavior in the subsequent filtration step. If,
on the other hand, the bed of polyure-thane foam is re-
generated in the same manner as the normal filtration step,
by flowing a stream of solids-depleted liquid downwardly
through the solids-loaded bed, it is generally difficult
to obtain sufficient removal of suspended solids from the
polyurethane particles to satisfactorily renew the bed
for the subsequent filtration step.
~aced wlth the foregoing difficulties in the
application of conventional back washing to the polyure-
- thane foam filtration bed, and in view of ~he extremely
low density of the polyurethane foam f~ltration medium
re~ative to conventional media, the prior art has proposed
10861
~ 3~D
varlous means for compression regeneration of the polyure-
thane foam ln the filtration bed. Such compression re-
generation generally involves the use of mechanical means
for "squeezing" the particles of polyurethane foam, there-
by "wringing out" the contained water along with the
deposited solids from the particles of polyurethane foam.
Although such method of regeneration is easily employed on
a laboratory or bench-scale filtration unit, the mechanical
complexity and attendant capital expense of means for
squeezing the foam particles in commercial scale polyure-
thane foam filtration beds has severely limited the utility
of polyurethane foam f~ltration beds in practice.
Accordingly, it is an object of the present -in-
vention to provide an improved process for removing de-
posited suspendéd solids from polyurethane partiCles in a
filtration bed of such particles.
It is another object of the invention to provide
such a process which requires only a very low volume of
regeneration liquid.
Other objects and advantages of the present
invention will be apparent from the ensuing disclosure
and appended claims.
SUMMARY OF THE INVENTION
This invention relates to a p~ocess for removal
- of sollds from a liquid containing suspended solids.
Tn t~e process of this invent~ on, a filtration
vessel containing a filter bed of discontinuous polyure-
thane particles is provided. The liquid containing sus-
pended solids is flowed through the filter bed in the
10~61
filtration vessel for deposition of the solids on the
polyurethane particles to form solids-depleted liquid
and sollds-depleted liquld is discharged from the filtra-
tion vessel, until the filter bed is at least partially
loaded with removed solids. The flow of liquid through
the filter bed is terminated. Deposited solids are re-
moved from the discontinuous polyurethane particles in the
filter bed, thereby renewing the filter bed for solids
removal from the liqu~d containing suspended solids.
In the improvement of the invention, the re-
moval of deposited solids from the discontinuous polyure-
thane particles in the filter bed is carried out by adding
at least one bed volume of regeneration liquid to the
filtration vessel and mechanically mixing the filter bed
of discontinuous polyurethane particles having the solids
deposited thereon in a total volume of liquid comprising
the regeneration liquid and the hold-up liquid in the
filter bed resulting from the termination of liquid flo~
through the ~ilter bed to cause deposited solids to dis-
engage from the discontinuous polyurethane particles and
pass into the total volume of liquid, thereby forming
solids-enriched liquid. The solids-enriched liquid is
discharged from the filtration vessel.
In one preferred embodiment of the present
invention the filter bed occupies no more than one-half
the volume of the filtration vessel, and the mechanical
mixing comprises mechanically agitating the filter bed of
polyurethane particles.
In another preferred embodiment of the present
invention, the regeneration liquid comprises between
10861
one and two bed volumes of liquid.
As used herein, "regeneration liquid" means liquid
which is able ~o take up in suspended form the deposited
solids which are disengaged from the filter bed of poly-
urethane particles during the mechanical mixing thereof. In
preferred practice, the regeneration liquid is substantially
free of suspended solids. As use~ herein, "hold-up liquid
in the filter bed" refers to the interstitial liquid be-
tween particles of polyurethane and to the liquid which is
retained in the pores of the polyurethane particles in the
filter bed subseque~t to termination of liquid flow through
the filter bed. "Bed volume" means a unit volume of liquid
which is equal to the volume of the filter bed
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic diagram of a filter bed
of diseontinuous polyurethane par~icles, such as may suit-
ably be used in the practice of the present invention.
.
Figure 2 is a schematic view of the Figure 1 filter
bed during the regeneration step.
Figure 3 is a schematic diagram of another
embodiment of the present invention, wherein a filter bed of
discontinuous polyurethane particles is disposed upstream
of a conventional sand filter.
1086 1
i31~
BRIEF DESCRIPTION OF THE PREFERRED EMBODI~ENTS
Referring now to Figure 1, liquid containing
suspended solids enters the treatment system in line 100
having control valve 115 disposed therein, which is open
during the initial phase of operation. This liquid may be
from a variety of sources9 as for example a biological
treatment plant effluent having 40 - 60 parts per million
suspended solids or river water for a municipal water treat-
ment facility. Alternatively, the liquid could comprise
raw water intended for boilers or for internal industrial
plant use or it could be a stream from an algae pond
(oxidation ditch) which is treated to prevent pin-point
algae flocs from reaching the receiving wa~ers. Fr~m line
100, the liquid enters the filtration vessel 102 and is dis-
charged in the upper end thereof through a spray distributor
or other discharge device 101. The filtration vessel 102
may suitably be constructed from carbon steel or other
structural material and is only partially filled with a
filter bed of discontinuous polyurethane particles 103, for
reasons which will be more fully apparent hereinafter. The
filtration vessel has an upper inlet end and a lower dis-
c~arge end. The filter bed of discontinuous polyurethane
particles is supported within the vessel by a lower support-
ing structure 104, which may suitably comprise a retaining
grid of screen or other fine mesh material. In ~his manner
10861
~ 3`~
a plenum space 10.5 is formed at the lower end of the vessel
102, for collection of the liquid which has been passed
through the filter bed.
The liquid containing suspended solids is flowed
through the filter bed in the filtration vessel from the
upper inlet end to the lower discharge end for deposition
of the solids from the liquid on the polyurethane particles,
to effect removal of the solids by the filter bed. Solids-
depleted liquid is thereby formed which collects ;n the
lower plenum space 105 and is discharged from the filtration
vessel in line 106, having open control valve 116 disposed
therein. From line 106, the solids-depleted liquid is
passed to receiving waters and/or other treatment and end
use.
The previously described flow of liquid containing
suspended solids is continued until the filter bed is at
least partially loaded with deposited solids, whereupon the
flow of liquid through the filter bed is terminated, and
the filter bed is ready for renewalO
In order to enhance the solids collection re-
moval efficiency of the filter bed in the previously des-
cribed system, it may be desirable in some instances to add
a coaugulant ma~erial, such as an organic polymer coa~ulant,
sodium silicate or alum, to the liquid being introduced to
the filter. Such coaugulants are in some applications
desirable in order to increase the flocculation of suspend-
ed solids in the liquid undergoing treatment and to reduce
~he turbidity of ~he final effluent discharged from the filter
bed treatment sys~em.
The polyurethane particles employed in the filter
bed of the present invention may be of any suitable foam
type having the requisite solids loading capacity for the
filtration step operating time employed. The discontinuous
polyurethane employed in the particles of the fil~er bed
may suitably be in the form of shredded non-uniformly sized
particles or, alternatively, ~he polyurethane particles in
the filter bed may have a cubic form, as may be obtained by
physically cutting a body of polyurethane foam into small
sized particles such as cubes having sides of 1/4 to 1 inch
length. Suitable polyure~hane foam materials which have
been employed successfully in practice have a cellular density
of from 10 ~o 100 cells/inch. The true density of the poly-
urethane particles in the filter bed should preferably be
greater than about 1.34 lbs./ft. in order to provide
structural integrity of the bed and to preclude floating of
the polyurethane particles during the filtration step The
polyurethane particles in the filter bed preferably have a
solids loading capacity of at least one-half pound suspend-
ed solids/ft polyurethane particles, in order to provide
suitably high capacity for solids loading during the filtra-
tion step. By way of example, a suitable foam polyurethane
- 10 -
;53~ 1
material for use in the process of the present invention
may be shredded in form, with particle lengths on the
order of 1 - 2 inches, 30 cells/linear inch, a particle
thickness on the order of 0,25 inch, a true density (measur-
ed without voids) of 1.77 lbs./ft. and a tensile strength
of 23 5 psi.
During the filtration step, the volumetric flow
velocity of the liquid containing suspended solids through
the filter bed is desirably maintained between 1 and 10
gal/minute/ft of filter bed cross-sectional area, in order
to provide adequate contacting of the liquid with the
polyurethane particles i~ the filter bed without the occurr-
ence of bypassing or other anomalous flow phenomens. After
the filter bed is at least partially loaded with deposited
solids, the flow of liquid through the filter bed is termin-
ated. Subsequently, deposîted solids are removed from the
discontinuous polyurethane particles in the filter bed,
thereby renewing the filter bed for solids removal from
the liquid containing suspended ~
,,-
.. _ _ _ _ _ .. .... . . , .. . , . .. . . ...... _ _ _ ........ .
lU~l
Prior to the renewal step, the flow of liquid
through the filter bed 103 is terminated, as for example by
closing the control valves 115 and 116 in lines 100 and 106 re-
spectively. At the termination of flow of liquid through
the filter bed, hold-up liquid will remain in the filter
103. In the renewal step, at least one bed volume of a
regenera~ion liquid is added to the filtration vessel 102,
to form a total volume of liquid in the filtration vessel
comprising the regeneration liquid and the hold-up liquid
in the filter bed resulting from ~he termination of liquid
flow through the filter bed. Figure 2 shows the sys~em of
Figure 1 wherein the filtration vessel contains the afore-
mentioned total volume of liquid. Prior to the addition
of regeneration liquid to the filtration vessel, the
filtration vessel will generally contain sufficient hold-up
liquid to constitute a major fraction of a bed volume of
liquid due ~o the high porosity of the fil~r bed of poly-
urethane particles. For example, the porosity of the filter
bed of polyurethane particles may be on the order of 80%,
whereby the filtration vessel at the termination of liquid
flow through the filter bed would contain 0.80 bed volume
of hold-up liquid as interstitial liquid between the part-
icles of polyurethane and liquid retained in the pores of
the polyurethane particles in the filter bed.
As stat~d earlier herein, the term "bed volume"
means a unit volume of liquid which is equal to the volume
. - 12
10861
~ 3~
of the f~lter ~ed Thus, if a fil~er bed is provided which
ls two feet ln height in the filtration vessel, the addition
of one bed volume of additional liquid (regeneration liquid)
would resul~ in the provision of an admixture of polyurethane
particles and liquid phase wherein the height of the liquid
in the filtration vessel would be 4 ft.
The regeneration liquid added to the filtration
vessel in the renewal step may comprise liquid containing
suspended solids which is introduced to the filtration
vessel in line 100, following which valve 115 in line 100
is closed. Alternatively, the regeneration liquid may
comprise solids-depleted liquid, as for example clean
water, when the liquid containing suspended solids is water.
The solids-depleted liquid may be introduced countercurrent-
ly into the filtration vessel following termination of
liquid flow through the filter bed by flow of the regenera-
tion liquid through line 113 and countercurrently through
that portion of line 106 which is directly benéath filtra-
tion vessel 102. In the latter case, valve 114 in line
113 is open, while valves 115 in line 100 and valve 116 in
line 106 are closed. During the normal filtration step,
valve 114 is closed, while valves 115 and 116 are open.
The switching of the various respective valves may be
carried out by a cycle time control system of a type well
known to those ~killed in the art.
- 13 -
~ 6
After at least one bed volume of regeneration
liquid has been added to the filtration vessel, as shown
in Figure 2, all valves 114, 115 and 116 are closed. In
preferred practice, the filter bed, as shown in Figure 1,
occupies no more than one-half the volume of the fil~ration
vessel, so as to allow for introduction of at least one bed
volume of regeneration liquid to the filtration vessel. In
the regeneratlon step, as mentioned, the filtration vessel
contains a total volume of liquid comprising the regenera-
tion liquid and the hold-up liquid in ~he filter bed re-
sulting from the termination of liquid flow through the
filter bed. In the general practice of the present in-
vention, the hold-up liquid in the filter subsequent to
the termination of liquid flow through the filter bed is
intended to be construed broadly so as to include hold-up
liquid in the filter bed, constituted by solids-depleted
liquid remaining in the bed following the termination of
liquid flow therethrough as well as to include the inter-
stitial liquid between particles of polyurethane and the
liquid which is re~ained in the pores of the polyurethane
particles in the filter bed when the filter vessel is drain-
ed of liquid following the termination of liquid flow through
the filter bed and the hold-up liquid is replaced by a
fractional bed volume of regeneration liquid. In other
words, it is contemplated in the broad practice of ~he
- 14 -
10861
~ 3~
present invention to completely drain $he filtration
vessel of liquid following the filtration step and then
to fill the fil~ration vessel with regeneration liquid to
the desired extent such that a portion of the total volume
of liquid replaces the hold-up liquid which was previously
contained between the particles of polyurethane and in the
pores of the polyurethane particles in the filter bed.
In this manner, the fractional bed volume of regeneration
liquid which fills the pores and interstices of the filter
bed becomes the hold-up liquid in the filter bed. Nonethe-
less, it is generally preferred in practice to retain the
hold-up liquid in the filter bed resulting from the termin-
ation o liquid flow through the filter bed in order to min-
imize the filter bed regeneration time and to simpl~fy the
regeneration procedure.
In the system as shown in Figure 2, after the
regeneration liquid had been introduced to the filtration
vessel to form a total volume of liquid comprising the re-
generation liquid and the hold-up liquid therein, the
filter bed of discontinuous polyurethane particles having
the solids deposited thereon is mechanically mixed to cause
deposited solids to disengage from the discontinuous poly-
urethane particles and pass into the ~otal liquid volume,
thereby forming solids-enriched liquid. This is carried
out by energizing the motor drive means 117 which is
- 15 -
1 ~ O ~ 1
~ 53~
connected by rotating shaft member 118 to rotating impeller
means 119. In this manner, the filter ~ed is mechanically
mixed by agitating the polyurethane particles in the total
liquid volume in the filtration vessel. Such agitation
causes disengagement of the deposited solids from the poly-
urethane particles in the bed and forms the aforemen~ioned
solids-enriched liquid in the filtration vessel. After a
suitable period of agitation, which for example may be
1 - 3 minutes,when disengagement of deposited solids from
the polyurethane particles has been effected, the motor
drive means 117 is deenergized and the solids-enriched
liquid is discharged from the filtration vessel in line 106
by opening of valve 116; The solids-enriched liquid in
line 106 may be passed to further treatment and/or end use
facilities, such as sludge drying beds. After the solids-
enriched liquid is fully discharged from the filtration
vessel, the filter bed is fully renewed for the subsequent
filtration s~ep, which may be initiated by opening valve
115 in feed liquid line 100 to admit liquid containing sus-
pended solids to the filtration vessel 102 for filtration
therein as earlier described.
It was stated earlier herein that the regenera-
tion liquid preferably comprises between one and two bed
volumes of liquid. Such volumetric amounts of regenera-
tion liquid are satisEactory in the broad practice of the
- 16 -
~ 3~
present invention to fully regenerate the polyurethane
particle filter bed. By way of example, experience has
shown that a 30 cell per linear inch polyurethane foam
filter bed can be satisfactorily regenerated in one bed
volume of regeneration liquid (in addition to the hold-up
liquid in the filtration vessel.) A 60 cell per linear
inch polyurethane foam filter bed, on the other hand, has
been found to require 1.6 bed volumes of regeneration
liquid (in addition to the hold-up liquid in the filtra-
tion vessel) for efficient regeneration. This difference
is due to the fact that the 60 cell polyurethane foam is
less dense than the 30 cell polyurethane foam and thus the
former requires a correspondingly greater volume for mechan-
ical mixing in the regeneration step.
Referring now to Figure 3, ~here is shown a
schematic diagram of another process system in which the
process of the instant invention can be carried out. Liquid
containing suspended solids is introduced into the system
in line 200 having con~rol valve 215 disposed therein from
which it is distributed by the distributor means 201 a~ the
upper end of the filtration vessel 202 to the filter bed 203
of discontinuous polyurethane particles contained in the
filtration vessel. The filter bed of discontinuous poly-
urethane particles has an u~per inlet end and a lower dis-
charge end, with the latter resting on support structure
204~ which provides a lower plenum space 205 for collec~ion
~ 53~
of purified llquid from the filter bed. The liquid contain
ing suspended solids is flowed through the filter bed in
the filtration vessel for deposi~ion of the solids on the
polyurethane particles to form solids-depleted liquid. The
solids-depleted liquid is discharged from the filtration
vessel 202 in line 206. During the normal filtration step
valve 215 in line 200 is open and valve 209 in line 208 is
closed.
From line 206, solids-depleted liquid is passed
to the downstream filter bed 237 for final effluent polish-
ing of the liquid. The do~lstream filter bed 237 may be a
conventional sand filter or of a mixed media type. The
downstream bed 237 is contained in vessel 236 and receives
liquid from line 206 through distributor means 23~, which
may be of a type similar to that employed in filtration
vessel 202. The liquid flowing through the downstream
filter bed 237 is discharged therefrom in line 238 and
passed to reeeiving waters and/or other desired end use.
The above-described filtration step is continued
until the filter bed is at least partially loaded with
deposited solids, at which point the flow of liquid through
the filter bed is terminated. The filter bed of polyurethane
particles is now ready for renewal by removal of deposited
solids from the discontinuous polyurethane particles of the
bed, thereby re-establishing the capability of the filter
bed for solids removal from the liquid cont~ining suspended
- 18 -
~ 3
solids.
In the renewal step, at least one bed volume of
regeneration liquid is added to the filtration vessel
202. The filter bed of discontinuous polyurethane particles
having the solids deposited thereon is then mechanically
mixed by agitation by energizing motor drive means 217
which is connected by shaft member 218 to impeller means
219. Mixing agitation is carried out for a suitable time,
as for example 1 - 3 minutes, in a total volume of liquid
comprising the regeneration liquid and the hold-up liquid
in the filter bed resulting from termination oE liquid
through the filter bed~ to cause deposited solids to dis-
engage rom the discontin~ous polyurethane particles and
pass into the total liquid volume, ~hereby forming solids-
enriched liquid. At this point valve 209 in line 208 is
opened and the solids-enriched liquid is discharge~ from
; the filtration vessel in line 208. Subsequent to the dis-
charge of solids-enriched liquid from the filtration vessel
202, the filter bed 203 in the vessel is renewed and ready
for the normal fil~ration step. The second filter bed 237
may be regenerated in any suitable manner such as by con-
ventional back-washing.
The advantages of the present invention are
illustrated in the ollowing example.
- 19 ~
10861
~ S3
EXAMPLE
A comparative performance evaluation was carried
out on filter beds regenerated by back-washing and by
agitation mixing in manner of the present invention. In
the evaluation, two filter beds were arranged in parallel
flow relationship and were arranged to filter identical
respective portions of a common flow of solids-containing
waste water. Each bed was contained in a glass column
having an inner diameter of 14 centimeters. Each con~ained
75 grams of a 60 cell per linear inch shredded polyurethane
foam of approximately 95% porosity which formed filter beds
in the respective columns each 23 centimeters in height,
at a packing density of 30 grams of polyurethane foam per
liter of filter bed volume.
The respective filter beds each processed a flow
of 200 milliliters of suspended solids containing waste
water per minute. Four separate runs were made with re-
generation between successive filtration steps and the
fourth run was considered as representative of steady
state operating conditions. Each filtration step was
carried out for 8 hours. Subsequent to the termination
of the filtration step, one of the filter beds was re-
generated by back-washing and the other was regenerated
by agitation mixing in the manner of the present invention.
In the back-washi~g regeneration step for the
- 20 -
~ 3~
first fil~er bed, clean water was flowed as the back-
wash medium through the filter bed in a direction counter-
current to the down flow direction of liquid flow through
the filter bed during the normal filtration step. In the
back-washing step, the back-washing flow rate was 18 gallons
per minute per ft. of filter bed cross-sectional area. Re-
generation was carried out for 5 minutes, at which ~ime the
effluent back-wash liquid appeared to be relatively solids-
iree .
In the filter bed regenerated in accordance with
the method of the present invention, one bed volume of
regeneration liquid ~clean water) was added to the filtra-
tion vessel and the filter bed was mechanically mixed in a
total volume of liquid comprising the one additional bed
volume of regeneration liquid and the hold-up liquid in the
filter bed resulting from the termination o~ liquid flow
through the filter bed. Following the addition of the re-
generation liquid, the filter bed was mixed for two minutes
by an impeller comprising four 45 pitched blades. The
diameter of ~he mixing impeller was approximately 5 centi-
meters and the impeller was rotated dur;ng the mixing agita~
tion s~ep at a rotstional speed on the order 100 - 200 rpm.
The mixing agitation was then terminated and the bed was
drained of liquid. The filter bed was then refilled wi~h
two additional bed volumes of clean water and again mechan-
ically mixed for two minutes. The mixing agitation was again
terminated and the bed returned to the filtration operation.
10861
~ 6
Comparative performance data for the respective
regener~ion modes are set forth below in Table I.
_ Table I _ _ _ _ _
Comparative Performance of Filter Beds
Regenerated by Back-washing and by Agitation
Mixin~
Parameter ~ D~tion Method
Backwashing Agitation Mixing
. . _ ,_
Total Regeneration
Liquid Volume, gal. 14 4.5
Solids initially on
filter bed PmS 18 18
_ , _
Solids on filter bed
after regeneration,
~ms 5 0O5
.~ . _ _ _
Solids re~oved, % 72% 97.5~O
As shown by the data, the backwashing regeneration method
required 14 gals. of total regenerativn liquid, whereas the
agitation mixing regeneration method of this invention re-
quired only 4.5 gals. (note: the total regeneration liquid
volume listed in the Table for the agitation mixing method
comprises the added bed volumes of regeneration liquid and
the hold-up liquid in the filter bed contained in ~he pores
and interstices of the filter bed~. As also shown by the
- data, the back-washing regeneration method yielded only a
72% solids removal from the filter bed, despite the large
volume of regeneration liquid employed, while the agitation
mixing regeneration method of the present invention resulted
- 22 -
in a 97.5/0 solids removal.
Although preferred embodiment of the invention
have been described in detail, it is to be recognized that
other embodiments only with modification of the disclosed
features are contemplated as being within the scope of the
invention.
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